1.10.3. Security

As stated earlier in this chapter, security cannot be an afterthought, and security under Red Hat Enterprise Linux is more than skin-deep. Authentication and access controls are deeply-integrated into the operating system and are based on designs gleaned from long experience in the UNIX community.
For authentication, Red Hat Enterprise Linux uses PAM -- Pluggable Authentication Modules. PAM makes it possible to fine-tune user authentication via the configuration of shared libraries that all PAM-aware applications use, all without requiring any changes to the applications themselves.
Access control under Red Hat Enterprise Linux uses traditional UNIX-style permissions (read, write, execute) against user, group, and "everyone else" classifications. Like UNIX, Red Hat Enterprise Linux also makes use of setuid and setgid bits to temporarily confer expanded access rights to processes running a particular program, based on the ownership of the program file. Of course, this makes it critical that any program to be run with setuid or setgid privileges must be carefully audited to ensure that no exploitable vulnerabilities exist.
Red Hat Enterprise Linux also includes support for access control lists. An access control list (ACL) is a construct that allows extremely fine-grained control over what users or groups may access a file or directory. For example, a file's permissions may restrict all access by anyone other than the file's owner, yet the file's ACL can be configured to allow only user bob to write and group finance to read the file.
Another aspect of security is being able to keep track of system activity. Red Hat Enterprise Linux makes extensive use of logging, both at a kernel and an application level. Logging is controlled by the system logging daemon syslogd, which can log system information locally (normally to files in the /var/log/ directory) or to a remote system (which acts as a dedicated log server for multiple computers.)
Intrusion detection sytems (IDS) are powerful tools for any Red Hat Enterprise Linux system administrator. An IDS makes it possible for system administrators to determine whether unauthorized changes were made to one or more systems. The overall design of the operating system itself includes IDS-like functionality.
Because Red Hat Enterprise Linux is installed using the RPM Package Manager (RPM), it is possible to use RPM to verify whether any changes have been made to the packages comprising the operating system. However, because RPM is primarily a package management tool, its abilities as an IDS are somewhat limited. Even so, it can be a good first step toward monitoring a Red Hat Enterprise Linux system for unauthorized modifications.